Diseases of Herbaceous Perennials

Diseases of

Herbaceous

Perennials

Mark L. Gleason, Margery L. Daughtrey, Ann R. Chase, Gary W. Moorman, and Daren S. Mueller

Preface

In the world of ornamental production and gardening, beauty is foremost. There are literally hundreds of books that tell us how to grow the prettiest yarrow, or why we should buy a beautiful campanula, or regale us with the low maintenance value of that lovely coneflower.

So it would appear to be difficult to get excited about a book whose topic is to show how bad those same plants can look. It would seem to be a challenge to expect a reader to want to learn about the dreadful things that can assault their favorite perennial, and then expect the same reader to look at photos of that damage as well. A bit like viewing a car wreck, some would say. Not so! That my plants are going to get mugged occasionally by diseases is not in question. The real questions are—do I know how to identify, prevent, or reduce the damage?

To believe that all perennials are perfect is to believe that all men can fix cars. Pathogens, given the right environment, result in leaf spots, root damage, and death. These authors have been at the forefront of the pathogen wars for decades and their common sense approach to plant disease management is highly respected in scientific, commercial and garden circles. They provide more of the same in this book.

I approached the book as a producer, a landscaper, and gardener. I wanted to know what pathogens might attack my Acanthus, Echinacea and hundreds of other perennials that I just have to have. However, since I have

no lab to evaluate leaf spots or root swellings, and I am not a plant pathologist, I also wanted good photos to verify problems I might see in my perennials. And, most important, I wanted some common sense ideas explaining how to reduce the incidence of these awful problems.

I have not been disappointed. For every plant, this book tells me what might occur, it shows me what these occurrences might look like, and best of all, provides sensible suggestions and scientifically proven solutions. I didn’t want to hear about chemicals because it is well known that chemicals that are here today may be gone tomorrow—and I want to use as few chemicals as possible. I can easily understand and implement suggestions such as “choose a better cultivar, this one is highly susceptible” or “dead heading is the best prevention,” or “simply provide wider spacing in the garden.” Common sense runs in the veins of the authors, and the book confirms it.

Diseases may not be fashionable topics in this world of perennial beauty, but they are real, and this book tackles the reality head on. I am excited—I recommend it without question.

Dr. Allan Armitage Professor of Horticulture University of Georgia Author: Herbaceous Perennial Plants, 3rd ed.

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About the Authors Mark Gleason is a professor and extension plant pathologist in the Department of Plant Pathology at Iowa State University, where he has focused on diseases of horticultural commodities, including ornamentals, for the past 24 years.

Margery Daughtrey is a senior extension associate with Cornell University’s Department of Plant Pathology and Plant-Microbe Biology. She has 30 years of experience in extension education and applied research on diseases of greenhouse and nursery crops at the Long Island Horticultural Research & Extension Center.

Ann Chase is the founder and CEO of Chase Horticultural Research, a private research and diagnostic firm located in Mt. Aukum, CA. She was professor of Plant Pathology at the University of Florida Central Florida Research and Education Center in Apopka from 1979 to 1994.

Gary Moorman is a professor in the Pennsylvania State University Dept. of Plant Pathology, where he has worked in woody ornamental and floriculture pathology research, extension and teaching for the past 27 years. Previously he was a member of the Department of Plant Pathology of the University of Massachusetts at the Suburban Experiment Station in Waltham, MA working on diseases of vegetables and floricultural crops from 1978 to 1982.

Daren Mueller has been with the Department of Plant Pathology, Iowa State University since 2003, and now serves as an extension program specialist. He has expertise on white mold and rust diseases of ornamental and field crops, including two invasive introduced diseases: daylily rust and Asian soybean rust.

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Acknowledgments

The authors would like to express special gratitude to Winnie Gleason, Robert Kent, Mike Zemke, Fran Moorman, and Katie Mueller for their patience and support during the writing and illustration-gathering for this book. We have visited many beautiful gardens and well-run nurseries to collect images of herbaceous perennials over the past several years, and this part of the work was indeed a pleasure. Your hospitality to our cameras has been deeply appreciated. Another special thank-you goes to Allan Armitage, whose excellent book, Herbaceous Perennial Plants —A Treatise on Their Identification, Culture, and Garden Attributes, now in its third edition, supplies such well-informed perspective on the growth of herbaceous perennials in different areas of the United States. Many other people also deserve special mention for their important contributions to this book. Janna Beckerman, Scott Clark, Stephanie Cohen, June Croon, Jim Glover, Caroline Kiang, Joseph LaForest, Ben Lockhart, Barry Menser, Janice Munson, Theo Overdevest, Leanne Pundt, Henriette Suhr, Ellen Talmage, Maria Tobiasz and Andre Viette, in particular, were amazingly helpful. A number of talented photographers who have shared their images with us are listed on page 255. This book couldn’t have been done without a lot of help from our colleagues and associates—so thanks to you all!

SPONSOR

Iowa Nurserymen’s Research Corporation We are especially grateful to the Iowa Nurserymen’s Research Corporation, whose invaluable financial support made this book possible.

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Contents

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Diagnosing and Managing Diseases of Herbaceous Perennials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 General Types of Diseases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Diseases by Host Plant Genus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Common-Latin Name Directory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Photo Credits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 ix

Introduction North American gardeners are planting a rapidly expanding range of herbaceous perennials.

New plants are flooding the marketplace, and formerly obscure plants are becoming widely available. Learning how to take care of so many new kinds of plants can be a challenge.

Dealing with plant diseases is part of this challenge.

Knowledge about diseases of herbaceous perennials is mushrooming almost as fast as the perennials industry, but is scattered among many plant pathologists, horticulturists, books, websites, extension publications, and scientific articles.

The goal of this book is to help you to recognize the major diseases of herbaceous perennial ornamentals and to manage them effectively. Our geographic focus is North America (the United States, Canada, and Mexico); diseases known elsewhere in the world will not necessarily be mentioned here. Because the ornamental plant trade today is a global industry, however, problems currently restricted to other continents may well be brought into North America in the future. Similarly, diseases known to occur on wild relatives of ornamentals may occasionally appear in the garden, so we have reported problems that occur on the host plant’s genus even if they have not been common on garden plants.

This book is intended for home gardeners, commercial plant producers and retailers, landscape managers, diagnosticians, educators, and anyone else who loves perennials.

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What is an herbaceous perennial?

We define herbaceous perennials as ornamental plants that lack woody tissue and are perennial (persist from year to year) in at least some part of North America. The book emphasizes perennials that are generally grown outdoors, rather than greenhouse or interiorscape species. Some of the plants covered are listed as “tropical perennials� because they are not frost-hardy.

How to use the book This Introduction is followed by a section describing basic strategies for diagnosing and managing diseases of herbaceous perennials. It explains why Integrated Pest Management is important, and outlines some helpful practices to keep diseases in check.

Next come short profiles of 12 major types of diseases that attack herbaceous perennials. This section will help readers understand how different types of diseasecausing agents live their lives, and how best to protect plants against them.

The main section of the book is organized alphabetically by the genus of perennial plant. To help readers rapidly find information for a particular plant of interest, the genera are organized alphabetically. If you are unfamiliar with Latin names of the plants you are interested in, check the Common Name/Latin Name Index on page 249 to find the Latin name of the plant you are seeking.

Photos of some of the major diseases of each genus are combined with text descriptions.

We have included as many genera of herbaceous perennials as possible. You may find that some plants that you grow do not appear in the book; this may be because because no diseases have been reported on these plants from North America.

Diagnosing and Managing Diseases of Herbaceous Perennials

Since this book focuses on the recognition and management of plant diseases, a few key ideas from the science of plant pathology will be helpful to the reader. Symptoms on plants may be caused by either non-contagious factors (such as cultural errors in watering or fertilization) or by microorganisms, which are contagious. Every contagious plant disease involves an interaction between three players: the host plant (the victim), the pathogen (the attacker), and the environment. Plants get sick only when a pathogen comes in contact with a plant that is susceptible to it, and then only when the environment allows the pathogen to attack.

Even though there are thousands of different species of plant pathogens — fungi, bacteria, nematodes, viruses, and even parasitic plants — deciding how to manage them is simpler than it looks. Luckily, each type of host plant — hosta, daylily, astilbe, etc. — is attacked by a relatively short list of pathogens. So once you identify your host plant, you have already taken a huge step toward making the diagnosis by reducing the number of possible diseases.

Another plus is that pathogens come in only a few types. The following section of this book acquaints you with 12 major pathogen types, and provides tips on managing each type. For example, although many species of powdery mildew fungi attack perennials, almost all powdery mildew species behave similarly. So once you understand how to manage powdery mildew on dahlia, the same general IPM approach is likely to work on delphinium — even though the species of powdery mildew fungi usually differ on plants from different plant families.

Learning the basic attack strategy used by each type of pathogen will give you a head start in deciding how to prevent the diseases they cause.

Diagnosing the cause of a plant disease puts you on the road to managing it successfully. Once you know what is causing the symptoms on your plants, you can zero in on the best ways to control the disease — and take steps to avoid it in the future. This book is intended to help you diagnose disease problems by matching what you see on your plants with the book’s photos and text descriptions. In the main body of the book, we have described the diseases of the most commonly planted genera of herbaceous perennials. Additional

text and Internet resources that can help with diagnoses are listed at the end of the book.

Extension specialists and plant diagnostic clinics across the United States provide invaluable local and regional expertise in diagnosis as well as treatment and prevention advice. It’s well worth your while to get acquainted with the perennial plant disease experts in your area, in order to learn how they can help you. But you don’t have to be a plant scientist to be good at diagnosis. Armed with a few well-illustrated resources, anyone who loves plants and is a careful observer of clues can become an excellent disease troubleshooter.

Being a grower of herbaceous perennials, whether for sale, public display, or personal pleasure, means making many decisions that affect the risk of disease outbreaks.

Which varieties you choose, the cultural practices you use to grow them, and which (if any) chemical or biological control products you apply can all affect the appearance of your plants and the cost of pest management.

Each decision a grower makes can influence the other decisions. For example, deciding to grow a variety that is

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very resistant to major diseases can cut the risk of having severe outbreaks of those diseases. On the other hand, growing a disease-prone variety because you love it, or because your buyers really want it, means that you may need to put extra effort into protecting it from diseases.

On the cultural side, it may be simpler or less expensive to use sprinklers rather than drip irrigation. But sprinklers can raise the risk of fungal and bacterial diseases that thrive on wet foliage, so you may sometimes need pesticide sprays to protect the plants.

How do you decide what disease management strategies make the most sense for you? Making sensible choices seems challenging when you are growing many different species of herbaceous perennials, each with its own growth characteristics and risks. To avoid getting

lost in the details, it’s helpful to rely on some basic disease management principles.

Integrated Pest Management (IPM) is a strategy for bringing together the most appropriate management practices — including cultural, genetic resistance, biological, mechanical, and chemical options — to keep diseases, insects, weeds, and other pests in check, with minimal harm to the environment and your checkbook.

An IPM plan should make sense for your particular environment, whether it is a commercial nursery, retail garden center, public landscape, or home garden. Likewise, disease management plans need to make sense alongside strategies for controlling insects and other pests, and for meeting the grower’s goals. We have described IPM strategies for disease management throughout this book.

Diseases by Host Plant Genus A Acanthus Acanthaceae

Acanthus, known by the colorful name of bear’s breeches, requires rich soils that are light and well-drained, and sunny locations in the garden. Plants can be killed by overly wet conditions during winter. Mulches can be important for helping winter survival in the north.

Plants in the genus Acanthus are rarely troubled by diseases, although slugs and snails may visit them in spite

of their formidable foliage. Acanthus species are susceptible to root knot nematode (Meloidogyne), which can cause small bump-like galls on the roots. Powdery mildew caused by a fungus and shoot proliferation caused by the bacterium Rhodococcus fascians (previously known as Corynebacterium fascians) have also been reported on Acanthus species but appear to be rare.

Distortion and adventitious bud development on the leaf of an acanthus infected with Rhodococcus fascians.

Clump of adventitious buds developing on acanthus stem infected with Rhodococcus fascians.

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Achillea Asteraceae/Compositae

Achillea, or yarrow, is a popular and reliably performing perennial. Many varieties and hybrids are available; the flowers vary in color from species to species. These plants appreciate sunny or partly shaded sites, and tolerate poor soils as long as drainage is good. If night temperatures are too warm the stems will topple, and some flowers will fade in hot weather. Dividing plants and replanting will rejuvenate them if they have ceased to flower well.

Yarrows are generally disease-free in favorable climates, but insects can cause some symptoms that may be mistaken for diseases. The foliage can be browned by feeding injury and droppings from lacebugs and thrips.

White foamy spittlebug masses are sometimes observed on stems. Few problems occur on yarrow during production and most can be controlled through judicious use of cultural and chemical controls.

A rust disease caused by Puccinia millefolii will cause chocolate brown bumps to appear on the leaves. Wet summer conditions favor this fungal disease, which is most often seen in the western and southwestern states. In wetter climates, Botrytis cinerea can attack flowers or stems, or leaf spots may occur after infection by species of the

Close-up views of chocolate-brown rust pustules on yarrow.

fungi Alternaria, Cercospora, Leptosphaeria, Pleospora, or Septoria. All of these are kept in check by careful attention to irrigation practices. Powdery mildew, crown gall and root knot nematode are also possibilities. Yarrows are sometimes troubled by lower stem cankers caused

by Rhizoctonia solani, especially if mulched too deeply.

Pythium root rot and Phymatotrichopsis root rot can also occur. Avoid excessive overhead irrigation of yarrow to prevent foliar and root rot disease problems.

Rust pustules on yarrow may be fairly inconspicuous because the leaves are so narrow.

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Aconitum Ranunculaceae

Aconitum, or monkshood, is a genus of plants with an aura of mystery about them due to their hooded flowers and the poisonous character of both the roots and above-ground parts. Monkshoods are not appropriate plants to grow adjacent to root crops in the vegetable garden—but these flowers have an unusual advantage in that they are left alone by deer, and their curiously hooded form is enchanting. The twopetaled flowers help to separate them from delphiniums, which have a similar leaf. They tolerate some later afternoon shade. Dry to moist (but not soggy) soils are acceptable, and richer soils are preferred. Aconitums are better adapted to growing in the northern United States or higher elevations in the south, as cool nights are best for their health. Winter mulching with a layer of leaves helps survival in the north.

If plants become crowded, their flowering will decline, so make divisions and replant every few years, in the fall. Plants may need a season to fully recover from transplanting.

Aconitums are quite susceptible to Verticillium wilt caused by Verticillium albo-atrum. The affected plants will perform poorly in the garden—their leaves will turn brown and dry, and flowering will be reduced. Cutting across the stem of an affected plant will reveal dark discoloration of the vascular bundles (water conducting tissues). If this disease is detected, do not try to replant monkshood in the same part of the garden, as the microsclerotia of the fungus will remain in the soil for several years to attack new specimens.

Monkshoods are susceptible to many of the same diseases as their close relatives, delphiniums. These include black leaf spots caused by Pseudomonas bacteria and the white colonies of moldy growth on leaves caused by the powdery mildew fungi. Foliar nematodes (Aphelenchoides) may cause discolored areas in the leaves. Stem rots may be caused by the fungi Sclerotinia sclerotiorum, Sclerotium rolfsii, S. delphinii and Rhizoctonia solani, and root rot caused by Phymatotrichopsis omnivora can cause problems in the extreme southern United States. Root knot nematode, rust, smut and downy mildew diseases have also been reported for Aconitum species. Aconitum

is susceptible to Impatiens necrotic spot virus (INSV), a thrips-borne virus that can cause a variety of symptoms including leaf spots and mottling of foliage, as well as Cucumber mosaic virus (CMV), which is spead by aphids and can cause mottling of leaves.

Swollen areas on the roots of monkshood are symptomatic of infection by root knot nematode, Meloidogyne hapla.

Mottling of monkshood leaves, caused by CMV.

Foliar nematode infestation (Aphelenchoides sp.) has caused the blackened, aborted sprout on this monkshood transplant.

The underground stems of monkshood show blackening of the vascular system due to Verticillium wilt.

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Actaea Ranunculaceae

Actaea (previously called Cimicifuga), known as cohosh or baneberry, features some poisonous parts—the roots and the handsome berries. These plants appreciate a woodland environment, so they should be grown in rich, moist soils with plenty of organic matter, in shade.

Similarly to astilbe, actaea will be subject to injury from dry periods in the summer and will need supplemental irrigation in many climates.

Few diseases are known for these plants. Some fungal leaf spots caused by species of Ascochyta, Ramularia, and Phyllosticta have been reported. Leaf spotting may also be caused by foliar nematodes, which cause brown patches between the veins when they feed within the leaf tissue.

Actaea spp. are susceptible to some rust fungi, including Puccinia recondita that has certain grasses as its alternate hosts. Smut diseases caused by species of Urocystis have also been reported. These foliar problems along with foliar nematode injury will be kept in check by minimizing periods of leaf wetness.

The globose white structures in the swellings on this actaea root are the swollen bodies of root knot nematode females (Meloidogyne).

Vein-limited necrotic areas in actaea leaves infested with foliar nematodes (Aphelenchoides).

Yellow mottling that indicates a possible virus infection on actaea.

A fungal leaf spot caused by an Ascochyta species on actaea.

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Aegopodium Apiaceae/Umbelliferae

The genus Aegopodium contains a single species, A. podagraria, whose common names include snow on the mountain and bishop’s weed.

The variegated form A. podagraria ‘Variegatum’ is not as vigorous as the species and appears to be especially susceptible to Septoria leaf spot, which is the most important disease on aegopodium. This fungus infection causes irregular brown spots or marginal scorched areas on leaves, and the dead areas usually are misinterpreted as being a consequence of sunscald. Close examination, however, will show the tiny rounded spore bodies (called pycnidia) on the upper surface of the dead spots. Long, thin, multiseptate spores ooze out from the pycnidia and will easily splash from plant to plant in massed groundcover plantings or in closely-spaced nursery containers. To minimize injury from Septoria leaf spot, overhead irrigation should be timed for early in the day so that the foliage has time to dry before

nightfall. Fungicides may be used in nurseries to reduce the incidence of these fungal infections. This is the only disease problem noted in the United States. Powdery mildew, downy mildew and a few leaf spots are reported from Central Europe, the United Kingdom or Asia.

Large scorched areas may develop where multiple Septoria leaf spots have coalesced on aegopodium leaves.

Fungal leaf spots caused by Septoria sp. are common on aegopodium.